For a number of years, the polymer coating industry has been engaged in substantial development programs involving the identification or synthesis of various photoinitiators which would permit high speed curing of the polymerizable formulations following exposure to sources of electromagnetic radiation. This has been particularly true in various segments of the coating industry where it has been desired to eliminate the volatile solvents required in many polymer coating processes because of the potential hazards or because of the cost of the equipment to handle the evolved solvent vapors. However, the industry has long recognized the necessity to provide polymerizable compositions which would exhibit relatively long pot life and/or shelf stability prior to exposure to the source of electromagnetic radiation.
In William R. Watt U.S. Pat. No. 3,794,576 granted Feb. 26, 1974, there are described and claimed certain highly advantageous epoxy coating formulations which combine the desired rheological properties with suitable pot life and rapid curing. The formulations therein utilize as the photoinitiator diazonium salts which decompose upon irradiation by ultraviolet light to produce rapid curing of the coating to a tack-free condition. As explained in detail therein, the desired rapid curing of the epoxy formulations requires that they contain at least about 15 percent by weight of an epoxidic ester with two epoxycycloalkyl groups.
Since the disclosure of the Watt Patent, there have appeared a number of patents and publications disclosing onium catalysts effective to replace the diazonium catalysts specifically described in the Watt Patent. These onium catalysts are described in detail in Barton U.S. Pat. No. 4,090,936 granted May 23, 1978, and Crivello U.S. Pat. Nos. 4,069,055 granted Jan. 17, 1978 and 4,058,401 granted Nov. 15, 1977. The mechanism and operation of the triaryl sulfonium salts is described in detail in Crivello et al "Triaryl Sulfonium Salts: A New Class of Photoinitiators for Cationic Polymerization", JOURNAL OR RADIATION CURING, Volume 5, pages 2, 10-11, January 1978, and "UV Curing: Science and Technology", edited by S. P. Pappas (Technology Marketing Corporation, Stamford, Connecticut).
Among the most popular of the sulfonium catalysts for epoxy polymerization and for other cationic polymerization is tri-phenylsulfonium hexafluorophosphate. Many of the recent studies have centered upon the use of this photoinitiator. Although tri-phenylsulfonium hexafluorophosphate offers certain advantages over the diazonium type catalysts from the standpoint of longer shelf life and rapid curing, it is generally a relatively expensive material which has a limited range of spectral sensitivity. Accordingly, there has continued to be a significant need for relatively low cost, high speed photoinitiators affording relatively broad spectral sensitivity to electromagnetic radiation and which would also exhibit highly desirable shelf life.
Accordingly, it is an object of the present invention to provide a novel process for producing a highly effective photoinitiator for cationic polymerization of various monomers.
It is also an object to provide such a process which enables utilization of relatively inexpensive and readily available reactants and which utilizes relatively simple procedures for reaction and for purification if so desired.
Another object is to provide a process in which such photoinitiators are utilized to polymerize cationically polymerizable monomer formulations rapidly and effectively.